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Functional connections between dorsal premotor cortex (PMd) and primary motor cortex (M1) have been revealed by paired-pulse transcranial magnetic stimulation (TMS). We tested if such connections would be modulated during a cognitive process (response selection) known to rely on those circuits. PMd-M1 TMS applied 75 ms after a cue to select a manual response facilitated motor-evoked potentials (MEPs). MEPs were facilitated at 50 ms in a control task of response execution, suggesting that PMd-M1 interactions at 75 ms are functionally specific to the process of response selection. At 100 ms, PMd-M1 TMS delayed choice reaction time (RT). Importantly, the MEP (at 75 ms) and the RT (at 100 ms) effects were correlated in a way that was hand-specific. When the response was made with the M1-contralateral hand, MEPs correlated with slower RTs. When the response was made with the M1-ipsilateral hand, MEPs correlated with faster RTs. Paired-pulse TMS confined to M1 did not produce these effects, confirming the causal influence of PMd inputs. This study shows that a response selection signal evolves in PMd early during the reaction period (75-100 ms), impacts on M1 and affects behaviour. Such interactions are temporally, anatomically and functionally specific, and have a causal role in choosing which movement to make.